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Difficulty: MediumAssumptions in Causal Arguments

Marine biologist: In the coastal waters of the Vespian Sea, significant increases in the ghost jellyfish population consistently occur shortly after periods of unusually heavy rainfall. Heavy rainfall washes large quantities of agricultural runoff into the sea, resulting in massive blooms of phytoplankton. Because phytoplankton is the primary food source for ghost jellyfish, we can conclude that these phytoplankton blooms are the cause of the periodic surges in the jellyfish population.

Which one of the following is an assumption on which the marine biologist's argument depends?

  1. A
    The ghost jellyfish does not have any natural predators in the Vespian Sea that would consume them in large numbers.
  2. Heavy rainfall does not alter the coastal water environment in other ways that serve as the actual trigger for the surges in the ghost jellyfish population.Answer
  3. C
    Any massive bloom of phytoplankton in the Vespian Sea will inevitably result in a proportional surge in the ghost jellyfish population.
  4. D
    Massive blooms of phytoplankton do not occur in the Vespian Sea during periods of lower-than-average rainfall.
  5. E
    The ghost jellyfish is the only marine organism in the Vespian Sea whose population increases in response to an abundance of phytoplankton.

Answer

The argument assumes that heavy rainfall does not alter the coastal water environment in other ways that serve as the actual trigger for the surges in the ghost jellyfish population.
The correct answer identifies a 'defender' assumption necessary for the causal conclusion. The author observes that rainfall precedes the jellyfish surges and concludes that the phytoplankton blooms caused by the rainfall are responsible. For this causal relationship to hold, we must assume that the rainfall itself (such as a change in salinity or temperature) is not the actual, direct cause of the surges. If we negate this option—meaning the rainfall DOES alter the environment in ways that act as the actual trigger—the argument's conclusion that the phytoplankton blooms are responsible is destroyed.

Step-by-Step Solution

1
Identify the core argument structure.
Premise 1: Heavy rainfall correlates with jellyfish surges. Premise 2: Heavy rainfall causes phytoplankton blooms. Conclusion: Phytoplankton blooms cause the jellyfish surges.
Understanding the exact causal chain claimed by the author is essential for finding logical gaps.
2
Identify the logical gap in the causal reasoning.
The author observes that Event A (rainfall) is followed by Event C (jellyfish surge). The author claims that Event A causes Event B (phytoplankton), and therefore Event B causes Event C. The gap is the possibility that Event A causes Event C directly or through an unstated Event D.
Causal arguments based on correlated events often fail to account for alternate mechanisms triggered by the same initial event.
3
Evaluate the options using the Negation Technique.
Negating the correct answer yields: 'Heavy rainfall DOES alter the environment in other ways that serve as the actual trigger for the surges.' If this were true, the conclusion that phytoplankton caused the surges would fall apart.
A necessary assumption must be fundamentally true for the argument to survive; negating it will destroy the conclusion.

Key Concept

Assumptions in Causal Arguments (Ruling out alternate causes)
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Assumptions in Causal Arguments

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